In recent years, the ink jet recording method has been used for various print-related fields such as photo printing, various printings, and particular printings of markings and filters as this method can form images easily and cheaply. This method can form photo-like images (equivalent to silver-salt photo images in quality) by using a recording device which ejects and controls fine ink dots, ink-jet ink which has improved its color reproduction range, durability, and ejection aptitude, and dedicated paper which has strikingly improved the ink absorptivity, the coloring ability of color materials, and surface glossiness. As just described, the improvement of the image quality of the ink jet recording method can be accomplished only when the recording device, the ink-jet ink (or simply abbreviated as ink), and the dedicated paper are all optimum.
However, the paper dedicated for the ink jet system is rather expensive and this increases the printing cost. To save the recording cost, a lot of attempts have been made to record images on non-dedicated recording media by the ink jet recording method. Specifically, these attempts use a phase-change ink jet recording method using wax ink which is solid at room temperature, a solvent-based ink jet recording method using ink which mainly contains a quick-drying organic solvent, or a UV ink jet recording method using UV-setting ink which is cross-linked by ultraviolet ray (UV) light after it is recorded.
Among the above methods, the UV ink jet recording method has come to the fore as it can record on recording media which are resin-made, comparatively odorless, quick-drying, and non-ink-absorbing in comparison with the solvent-based ink jet recording method.
However, as described in Non-Patent Document 1, the UV ink jet recording method has various problems to be solved. Although Patent Documents 1 to 3 disclose UV-setting inks, under the circumstances, the inks are hardly adequate in terms of adhesiveness to various support members, sensitivity levels, stability of ink preservation, and stability of ink ejection, etc.
The UV-setting inks are loosely divided into two solventless UV ink which hardly contains non-hardening solvent and water-based UV ink in which a photo-induced polymeric compound is dissolved or dispersed in a water-based medium.
The solventless UV ink has merits of fast recording and already been in practical use since the ink can be hardened and dried simply by exposure to UV light and that the ink produces no harmful substance such as VOC. However, the solventless UV ink does not shrink during hardening and leaves mounds of ink fixed on the recording paper. This unlevelling can be smoothed to some extent by optimization of UV exposure timing and improvement of ink properties, but dot blurring and excessive glossiness will make the printout different from what is obtained by regular printing.
Meanwhile, the water-based UV ink has features that the water-based UV ink can use polymeric compound of higher viscosity than the solventless UV ink since the water-based UV ink using a water-based medium is diluted by a water-based solvent, that the water-based UV ink can reduce the ink volume and improve the image quality on the base material by evaporating the water medium from the UV-set ink, and that the water-based UV ink is available to the thermal type ink jet recording method. Generally, radical polymeric compounds can be widely used for the water-based UV ink. It is well known that active species (or radicals) in a radical reaction are quenched by oxygen.
As the solventless UV ink is less viscous than general UV-setting paint and the like, the ink has high dispersibility of oxygen into the ink. Therefore, attempts have been made to produce active species faster than oxygen disperses by intensifying the UV illumination and increasing the concentration of the photo initiator.
The water-based UV ink has an advantage in terms of sensitivity over the solventless UV ink since the water-based UV ink mainly uses water as the medium, has less dissolved oxygen than a hydrophobic organic solvent, and is hard to be inhibited.
Dissolved air and bubble cores (if any) in the normal water-based ink-jet ink will cause cavitations or the like when the ink is compressed and depressed repeatedly in the ink jet nozzle. To prevent this, attempts are made to remove the dissolved air from the ink as much as possible.
However, after careful studies and experiments, it is found that the water-based UV ink containing almost no dissolved air would not be ejected successfully after a long recording and exposure to UV light.
[Non-Patent Document 1]
Color materials, 2002, Vol. 75, No. 8, p.394-p.400
[Patent Document 1]
Tokukou H05-54667
[Patent Document 2]
Japanese Non-Examined Patent Publication H06-200204
[Patent Document 3]
Tokuhyou 2000-504778